Process of milling frangible material



April 22, 1952 swlNEHART 2,593,811

I PROCESS OF MILLING FRANGIBLE MATERIAL Filed NOV. 22, 1946 INVENTOR.

FRANK SWINEHART nected a motor II and a movable bed l2.

Patented Apr. 22, 1952 PROCESS OF MILLING FRANGIBLE MATERIAL Frank Swinehart, Strongsville, Ohio, assignor to The Brush Development Company, Cleveland, I

Ohio, a corporation of Ohio Application November 22, 1946, Serial No. 711,613

2 Claims.

My invention pertains to milling a frangible plate of piezoelectric crystalline material.

An object of my invention is to reduce scrap loss in the milling of piezoelectric plates.

A further object is to improve the quality of milled piezoelectric plates.

Still another object of my invention is to increase the life of the cutters used in milling piezoelectric plates.

It is also an object of my invention to mill closer to its final dimension a piezoelectric plate which is to be ground optically flat.

A further object of my invention is to provide a milled piezoelectric plate which has better surface finish than was hitherto possible.

And still another object of my invention is to provide a method of milling which permits the production of a thinner multiplate unit.

Other objects and a fuller understanding of my invention may be had by referring to the following description, claims and drawings wherein:

Figure 1 is a top view of a prior art crystal milling machine.

Figure 2 is a top view showing a milling cutter in position to edge m-ill a crystal in accordance with my invention.

Figure 3 is a bottom view of Figure 2.

Figure 4 is a top view of another milling cutter in position to face mill 2. crystal plate in accordance with my invention, and

Figure 5 is a bottom view of a milling cutter in position to face mill a different shaped crystal plate in accordance with my invention.

Figure 1 shows a milling machine comprised of a base member ID to which is suitably con- The motor drives a chuck I3 into which is connected a milling cutter I4 in the well-known manner.

' The cutter I4 extends out over the movable bed and the height of the bed with respect to the cutter is adjustable by means not shown. The bed I2 may be moved back and forth in the direction of the double-headed arrow by the handle I5-which'operates a gear in engagement-with a rack underneath the bed, or by any otherof the well-known mechanisms,- the details of which are not shown as they do not form part of my invention. The bed I2 may carry or include any of the well-known means 'for holding in place an article such as a crystal plate to be cut by the miller I4.

In the past frangible piezoelectric crystal plates ofthe Rochelle salt and primary ammonium phosphate types have been edge and face milled by the apparatus shown in Figure l with two of the edges of the plate I6 perpendicular and two of them parallel to the longitudinal axis of the cutter I4, as is shown in Figure 1. By keeping the cutter very sharp and by careful adjustment of the depth of the cut and the rate of feed by the handle I5 the crystal plates were successfully milled. There was, however, always a high rate of spoilage due to the crystal edges marked (A) and (B) being chipped. The loss was accepted because the operation was fairly fast and close dimensional tolerances of the plate could be maintained. Further, it was not required for some transducer uses that the plates be absolutely free of edge'chips; but even so, many plates were completely spoiled.

With the advent of the use of piezoelectric crystal plates in electro-optic devices wherein the light passes through the crystal, it has become necessary to provide crystal plates which are free from edge chips. Previous to my invention the crystal plates were edge and face milled to a dimension several hundredths of an inch greater in the thickness direction than the final desired dimension and only the best of the plates were then rough-ground and polished down to their desired thickness. By taking off several hundredths of an inch in the grinding and polishing operations the edge chips caused by the milling operation were ground and polished away. It of course takes a long time to remove several hundredths of an inch of crystalline material by grinding and polishing.

With my invention it is possible to mill the crystal plates down to within about .005 of an inch of their final dimensionbecause the edge chips are greatly reduced in number and those that are present are very small in siz'e'compared to those produced by the old milling method. Thus much less time is needed in the grinding and polishing operations. An added advantage which is realized for both electro-optic and transducer plates is that a deeper and faster cut can be made when using my invention than under the old practice. 1

Figure 2 shows howI reduce edge'chipping. The cutter I4 is not shown connectedi'nto the chuck of the motor as it was in Figure 1, it"being understood that the equipment illustrated in Figure 1 is utilized in Figures 2 to 5 inclusive. With the spiral cutter I4 rotating in either direction the crystal plate I6 is fed under it in the direction of the arrow I! or the arrow I8. When one cutting edge of the cutter I 4 first engages the face of the crystal to be milled, it is in engagement at only one point; however, as the cutting edge removes crystalline material from the plate, the cutting edge probably engages the crystal alon a line. If the cutter I4 is a 30 degree spiral cutter as shown, meaning that a tangent to a flute at the point where it is in engagement with the crystal plate makesa 30 degree angle to the longitudinal axis of the, cutter, then the plate should makea 60 degree angle to the longitudinal axis of the cutter. If the cutter is a. 45 degree spiral cutter, as shown in Figures 4 and 5, then the plate should make an angle of 45 degrees to the longitudinal axis of the cutter. some slight edge chippingmay occur at the end (B) but this is considerably reduced from that producedv by'the. prior'method, and the chipping at the other edges is reduced to zero.

When the crystal plate and cutter, in operative relation to each other, are viewed from underneath, as in Figures 3 and 5, it will be seen that the longitudinal axis of the plate is perpendicular to a line tangent to the flute at the point where the flute engages the crystal plate.

Some crystal plates are of shapes other than rectangular, such for example asthe trapezoidal plate 19 shown in Figure 5. When milling such a crystal minimum edge chipping will be obtained by having the longitudinal axis of the plate perpendicular to a line tangent to a flute at the point where thev flute is in engagement with the crystal, and by feeding the crystal into the cutter with such orientation that the direction of the force exerted by the cutter on the crystal is toward the broader end of the trapezoidal face.

The minimum edge chipping is obtained when the previously described relations are adhered to. However, experimentation has shown edgechipping is greatly reduced at any angle within about 5 degrees of the optimum set forth, and that deviations of greater than about 5 degrees from the optimum cause severe edge chipping.

The, relations set forth hold true whether the cutter is a right or a left-hand spiral, and are independent of the cutter speed as long as the cuttergis driven sufficiently fast to cut the material being worked. The relations hold true for all spiral cutter angles from the steepest down to the shallowest.

Observations over a considerable period of time during which a large number of crystal plates were edge and face milled show not only that the crystals are much better due to lack of edge chips but that my invention has more than doubled the cutter life. This unexpected benefit may be due to the fact that previously only very sharp cutters could, be used and now the cutters do not have to be quite so sharp yet yield crystals with good clean edges.

It is probable that the beneficialresults derived from the invention are due to the fact that the resultant of the important cutter forces is substantially parallel to the length dimension of the face of the crystal plate which is being milled. This means that there is substantially no force being exerted at an angle to the two long edges of the face being milled tending to break away edge chips.

In Fig. 1, which shows the oldpractice, as the crystal plate I S moves under the cutter N there are two important force components established which tend to edge-chip the crystal plate. One is due to the cutter tending to pull the plate underneath itself or to push the plate away, depending upon whether the cutter is cutting down into i the plate or not. The other force component is due to the rotating spiral teeth tending to thrust the crystal plate to the side. Other force components may be present but they are small compared to the above two important ones and so can be neglected. One of the unimportanttforces is the force feeding'the plate into the cutter, and another is the upward or downward thrust of the cutter on the plate. These forces are small so long as the operator actuates the handle I5 sufficiently slowly to prevent jamming the plate [5 against cutter M. In other words, so long as the operator operates. the equipment at a reasonable rate of speed the force components due to feeding the crystal plate under the cutter and the thrust can be neglected.

In accordance with the invention the optimum angle that the length dimensionof the crystal plate should make with the axis of the cutter is equal to the angle, which, the resultant of the important forces makes with the axisof the cutter. The direction of this resultant is dependent upon the degree of spiral of the. cutter andif this degree of spiral is subtracted from 90 degrees it gives the preferred angle between the axis. of the cutter and the direction. of. the length dimension of the plate being out. For example withya 45 degree spiral cutter the length direction of the plate should be at about 45 degrees to the axis of the cutter, as shown in Figs. 4 and 5. With a 30 degree spiral cutter the plate should be at about 60 degrees, as shown in Figs. 2 and 3. And for a 15 degree cutter the angle should be about degrees.

While I have described my invention with a certain degree of particularity, it is to be understood that changes can be madewithout departing from the spirit and scope thereof.

I claim as my invention:

1. The process of milling a plate of frangible piezoelectric material which comprises the steps of: orienting the plate with respect to a spiral milling cutter so that the angle between the direction of the length dimension of the face of the plate being cut and the axis of the cutter plus the spiral angle of the said cutter is between about and degrees, rotating said cutter, and with said plate so oriented with respect to the cutter effecting relative motion between said plate and said cutter in a direction at right angles to the axis of said milling cutter with said cutter in cutting engagement with said plate.

2. The process of milling a plate of frangible material as set forth in claim 1, further characterized in that said plate is trapezoidal in shape, and further characterized, by the step of orienting said trapezoidal plate to cause the forces exerted on said plate by said cutter during the cutting operation to be directed toward the broader end of said plate.

FRANK SWINEHART.

REFERENCES CITED The following referencesv are of record in the file of this patent:

UNITED STATES PATENTS 

